The invention relates to a protected length measurement system for measuring relative movements between a first machine part and a second machine part, which system comprises a sensing head and a rule sensed by it, said sensing head being securely mountable on the first machine part and said rule being connectable with the second machine part during final assembly of the length measurement system. The invention further relates to a method of final assembly of such length measurement system.
Length measurement systems for measuring relative movements between a first and a second machine part are known and are often used in machine tools, for example in order to sense the movement of a machine head or of a machine carriage relative to a machine bench. For this purpose, use is conventionally made of a rule which is mounted on the machine bench. A sensing head senses the rule, which may be effected in a non-contacting as well as in a contacting manner. However, because of their higher precision of measurement, optical, and thus non-contacting, sensing principles have found acceptance.
Length measurement systems of the aforementioned type may be divided into two categories: open and encapsulated measurement systems. In open measurement systems, the rule is openly mounted on the machine bench, and the sensing head mounted on the machine head moves above the rule. In encapsulated measurement systems, the rule is located in a closed capsule, in which the sensing head is also located. The sensing head runs on guiding elements located within the capsule and is connected with the machine head via a linking element so as to move along with it. An encapsulated measurement system is shown, for example, in DE 3,020,003 A1.
While encapsulated measurement systems achieve greater protection than open systems of the rule and of the sensing head against soiling, they do so at the cost of a great constructional complexity. Encapsulated measurement systems are accordingly more expensive than open measurement systems.
The mounting of a length measurement system on a machine, for example, on a machine tool, requires adjustment to the particular conditions present. The positions of the mounting spots for the rule as well as for the sensing head can be specified only within certain tolerances. For example, a certain tolerance is given with regard to the angle at which the mounting surfaces for the rule and for the sensing head are located relative to each other. In this connection, encapsulated measurement systems are advantageous because the sensing head, being guided within the capsule, is not connected directly with the machine head, but via a relatively easy to modify linking element. Said linking element needs to have as little clearance as possible in the direction of measurement. In the other directions, especially along the axis of the linking element, there is a certain flexibility simply because, due to the guiding means for the sensing head in the capsule, the distance between the machine head and the guiding means in the capsule may vary somewhat along the measured path to be covered.
In open systems, whose sensing head is usually securely attached to the machine head, no such flexibility is present, so that they require greater precision in terms of the relative positions of the mounting surfaces for the sensing head and the rule. This makes final assembly more difficult.
Therefore, it is the object of the invention to provide a length measurement system which achieves improved protection of the sensing head and of the rule with reduced technical complexity.